When certain electrical apparatus, such as transformers, reactors and shunt capacitors, are switched into a high voltage network by a circuit breaker which closes the circuit at an arbitrary point on the voltage wave, transient inrush currents with a large amplitude and rate of rise may occur. These currents may be detrimental to the apparatus and may cause mains interference.
To reduce the above-mentioned inrush currents, it is known to use a circuit breaker with closing (preinsertion) resistors, whereby the apparatus in question is first switched into the network via these resistors, which are thereafter short-circuited by the main contacts of the circuit breaker. However, this is a relatively expensive solution, which requires a complicated operating mechanism, which in turn involves reduced reliability.
It is further known to provide synchronous closing of apparatus of the above-mentioned kind with the aid of an electronic control device which gives a closing impulse to the operating device of the circuit breaker at such a time that the contact make in the three phases takes place at that point on the respective phase voltage wave which is most favourable in view of the closing transients. This means that transformers and reactors are switched in at the peak value of the respective phase voltage, whereas shunt capacitors are switched in at the passage through zero of the respective phase voltage. Equipment of this kind is described in IEEE Transactions on Power Apparatus and Systems, Vol. PAS-104, No. 9, September 1985 (R. W. Alexander: "Synchronous closing control for shunt capacitors"). In this equipment electronic tripping is provided in each individual phase. This requires a separate operating device for each breaker pole, i.e. altogether three operating devices.